1. Field of the Invention
The invention pertains to data communications systems, and particularly to diversity handover (e.g., soft handover) in a telecommunications system such as a wideband code division multiple access telecommunications system.
2. Related Art and other Considerations
In a typical cellular radio system, mobile stations (MS), also known as mobile user equipment units (UEs), communicate via a radio access network (RAN) to one or more core networks. The mobile stations (MSs)/user equipment units (UEs) can be mobile telephones (“cellular” telephones) and laptops with mobile termination, and thus can be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data with radio access network.
The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by a unique identity, which is broadcast in the cell. The base stations communicate over the air interface (e.g., radio frequencies) with the mobile stations within range of the base stations. In the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a radio network controller (RNC). The radio network controller, also sometimes termed a base station controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
One example of a radio access network is the Universal Mobile Telecommunications (UMTS) Terrestrial Radio Access Network (UTRAN). The UTRAN is a third generation system which in some respects builds upon the radio access technology known as Global System for Mobile communications (GSM) developed in Europe. UTRAN is essentially a wideband code division multiple access (W-CDMA) system. An undertaking known as the Third Generation Partnership Project (3GPPP) has endeavored to evolve further UTRAN and GSM-based radio access network technologies.
As those skilled in the art appreciate, in W-CDMA technology a common frequency band allows simultaneous communication between a mobile station (MS) and plural base stations. Signals occupying the common frequency band are discriminated at the receiving station through spread spectrum CDMA waveform properties based on the use of a high speed, pseudo-noise (PN) code. These high speed PN codes are used to modulate signals transmitted from the base stations and the user equipment units (UEs). Transmitter stations using different PN codes (or a PN code offset in time) produce signals that can be separately demodulated at a receiving station. The high speed PN modulation also allows the receiving station to advantageously generate a received signal from a single transmitting station by combining several distinct propagation paths of the transmitted signal. In CDMA, therefore, a mobile station (MS) need not switch frequency when handoff of a connection is made from one cell to another. As a result, a destination cell can support a connection to a mobile station (MS) at the same time the origination cell continues to service the connection. Since the mobile station (MS) is always communicating through at least one cell during handover, there is no disruption to the call. Hence, the term “soft handover.” In contrast to hard handover, soft handover is a “make-before-break” switching operation.
Direct sequence code division multiple access (DS-CDMA) thus allows signals to overlap in both time and frequency so that CDMA signals from multiple users simultaneously operate in the same frequency band or spectrum. In principle, a source information digital data stream to be transmitted is impressed upon a much higher rate data stream generated by a pseudo-random noise (PN) code generator. This combining of a higher bit rate code signal with a lower bit rate data information stream “spreads” the bandwidth of the information data stream. Each information data stream is allocated a unique PN or spreading code (or a PN code having a unique offset in time) to produce a signal that can be separately received at a receiving station. From a received composite signal of multiple, differently-coded signals, a PN coded information signal is isolated and demodulated by correlating the composite signal with the specific PN spreading code associated with that PN coded information signal. This inverse, de-spreading operation “compresses” the received signal to permit recovery of the original data signal and at the same time suppresses interference from other users.
In addition to receiving signals transmitted from several different transmitting information sources, a receiver may also receive multiple, distinct propagation paths of the same signal transmitted from a single transmitter source. One characteristic of such a multipath channel is an introduced time spread. For example, if an ideal pulse is transmitted over a multipath channel, the corresponding signal appears at the receiver as a stream of pulses, each pulse or path having a corresponding different time delay, as well as different amplitude and phase. Such a complex received signal is usually referred to as the channel impulse response (CIR).
A CDMA receiver employs a multipath search processor that searches for and identifies the strongest multipaths along with their corresponding time delays. A RAKE demodulator captures most of the received signal energy by allocating a number of parallel demodulators (called RAKE “fingers”) to the strongest multipath components of the received multipath signal as determined by the multipath search processor. The RAKE finger outputs are diversity-combined, after corresponding delay compensation, to generate a “best” demodulated signal that considerably improves the quality and reliability of communications in a CDMA cellular radio communications system.
The multipath search processor, (sometimes referred to herein as simply a “searcher”), identifies the channel impulse response of a complex received signal in order to extract the relative delays of various multipath components. The searcher also tracks changing propagation conditions resulting from movement of the mobile station or some other object associated with one of the multipaths to adjust the extracted delays accordingly.
More specifically, the channel impulse response of a received multipath signal is estimated within a certain range of path arrival times or path arrival delays called a “search window.” All signals detected within the search window form the delay profile, but only those signals originated by the transmitter belong to the channel impulse response. The remaining received signals in the delay profile are noise and interference. When the signals forming the delay profile are represented by their respective powers and delays, the delay profile is called a power delay profile (PDP).
Space diversity is attained by providing multiple signal paths through simultaneous links from a mobile station through two or more base stations. When the mobile station is in communication with two or more base stations, a single signal for the end user is created from the signals from each base station. As mentioned above, this diversity communication is sometimes referred to as a “soft” handover in that communication with a destination base station is established before communication with the source base station is terminated. Thus, after a call is initiated and established between a mobile station and a serving base station, the mobile station continues to scan a broadcast signal transmitted by base stations located in neighboring cells. Broadcast signal scanning continues in order to determine if one of the neighboring base station transmitted signals is strong enough for a handover to be initiated. If so, this determination is provided to the radio network which sends the appropriate information to the mobile station and to the new destination base station to initiate the diversity handover. The new base station searches for and finds the mobile station's transmitted signal using the associated spreading code. The destination base station also begins transmitting a downlink signal to the mobile station using the appropriate spreading code. The mobile station searches for this downlink signal and sends a confirmation when it has been received.
Diversity handover requires timing synchronization between the source and destination base stations and the mobile station. Synchronization should be achieved as rapidly and as simply as possible. In the downlink direction (from the base station to the mobile station), the mobile station locates and uses a known pilot signal contained in the base station broadcast channels to temporarily synchronize with the radio network system time. In the uplink direction (from the mobile station to the base station), a known pilot signal transmitted from the mobile station permits the source base station to estimate the channel impulse response for the uplink channel. Using this channel impulse response, the source base station derives synchronization signals necessary to extract the known pilot symbols from the received signal samples. Initial synchronization process occurs after the mobile station performs a random access over an uplink random access channel to acquire a traffic channel from the base station. At the completion of a successful random access procedure, the source base station is synchronized to the first arrived and detected multipath signal component originated by the mobile station and thereafter extracts pilot symbols later transmitted by the mobile station on the uplink traffic channel. For the W-CDMA context, radio interface synchronization in general is described in 3GPP TS 25.402. V3.3.0 (2000-09), which is the Technical Specification Group Radio Access Network, Synchronization in UTRAN Stage 2 (Release 1999) of the 3rd Generation Partnership Project.
During the synchronization procedure for the destination base station, a difficulty arises because there is an unknown propagation delay from the destination base station to the mobile station, and an unknown propagation delay from the mobile station to the destination base station. The sum of these propagation delays is called the round-trip delay, and it determines the delay between the transmit timing of the destination base station and the time when the signal is received at the mobile station. Namely, the mobile station receives the signal transmitted from the destination base station after a certain propagation delay from the instant when the signal is transmitted. The transmitted signal from the mobile station is synchronized with the received signal at the mobile station, so the transmitted signal from mobile station is delayed with respect to the base station transmission. The additional propagation delay from mobile station to the base station makes the delay of the received signal at the base station equal to the round-trip propagation delay.
The round-trip propagation delay is unknown in diversity handover because there is no random access uplink channel communication between the mobile station and the destination base station like there was with the source base station when the call connection was initially established. During the random access process, the propagation delay between the source base station and the mobile station is measured and used to facilitate the source base station synchronization. Since the round-trip delay between the mobile and destination base station is unknown, the searcher in the destination base station must scan all possible multipaths that could be generated by the mobile station located anywhere in the cell corresponding to the destination base station.
Since maximum delay of the received signal from the mobile station is unknown, a longer search window may be used to cover the maximum possible round-trip propagation delay, which corresponds to the destination base station cell size. As an example, a base station cell having a ten kilometer radius would have a corresponding maximum round-trip propagation delay of approximately eighty microseconds. A typical search window used in the source base station is on the order of ten microseconds. However, the search window in the destination base station would need to be eight times longer in order to accommodate the 80 microsecond propagation delay for this ten kilometer radius cell. Such a long search window is undesirable because of the increased data processing and memory resources required to perform the larger number of search and demodulation operations associated therewith. This large number of operations means increased synchronization delays. A longer search window therefore lessens the ability of the destination base station to respond to changes in the radio channel which translates, ultimately, into increased bit errors in the RAKE receiver outputs.
What is needed therefore, and an object of the present invention, is a technique which provides rapid synchronization of the destination base station receiver to the mobile station's uplink transmission in a diversity handover situation.